Transplantation of BM cells transduced with a retrovirus carrying the BCR-ABL fusion cDNA induced hematologic malignancies in about half of the recipients: either a CML-like myeloproliferative syndrome, acute lymphoblastic leukaemia (ALL) or tumours containing macrophage-like cells occurring after mean latencies of 9, 14 and 16.5 weeks respectively. The gene discussed is BCR; the disease is acute lymphoblastic leukemia.